Noise Control Questions

We have listed below some of the most common questions and their answers in hopes of providing basic knowledge related to noise, and noise control products and systems.
What is Noise?
At what level does noise become harmful to me?
What is dB-A?
What does 85 dB-A sound like?
What is N.R.C.?
What is STC?
How do we prevent equipment from overheating in a noise enclosure?
How do we maintain access to the equipment for maintenance?
Why is the machinery louder in my shop than what the manufacturer’s data shows?
Why do I care about noise at my property line?
What is Noise?
Noise is defined as unwanted sound. Unwanted sound can be hazardous to your hearing or can be simply disturbing, interfering, or annoying. Sound does not have to be loud to be unwanted.
At what level does noise become harmful to me?
Driven by OSHA standards, corporate safety departments, and insurance carriers, noise control levels are typically set at 85 dB-A as the high noise level. Individuals exposed to this noise are permitted to work an 8-hour shift with hearing protection. Administrative and engineered controls should be taken to reduce the employee’s noise exposure at 85 dB-A.
What is dB-A?
The sound pressure level is designed to closely reflect the response of the human ear. We are less sensitive to low and high frequencies, thus an “A” weighted noise level is what we hear.
What does 85 dB-A sound like?
Buses, motorcycles, and pneumatic tools at 50 feet. You must raise your voice to a near shout to speak over 85 dB-A.
What is N.R.C.?
The Noise Reduction Coefficient of a product is the average absorption across four-octave band center frequencies. (250 Hz., 500 Hz., 1000 Hz., 2000 Hz.) You can roughly estimate that a product with an NRC .75 will absorb 75% of the sound energy that hits it. The highest level is NRC 1.0.
What is STC?
A single number decibel rating of the transmission loss properties of a product. Doors, windows, walls, floors, etc. are tested to determine how much noise passes through. The testing determines a product’s STC. The higher the number the better.
How do we prevent equipment from overheating in a noise enclosure?
Silenced ventilation systems can be as simple as a blower system that moves existing plant air through the enclosure or as complex as separate HVAC systems. Silencers on the intake and exhaust will prevent the passage of noise in or out of the enclosure.
How do we maintain access to the equipment for maintenance?
Maintenance access is a key consideration in the design of an enclosure. Lengthy discussions with the equipment operators and maintenance personnel will ensure proper access is provided to all required areas. Swing, sliding, and removable doors and plugs are how most access is obtained.
Why is the machinery louder in my shop than what the manufacturer’s data shows?
Most equipment is tested in a free field condition, a room with no hard surfaces that reflect sound. Most industrial facilities have hard floors, hard walls, and hard ceilings. The reflected sound can build up to create a higher noise level. When the equipment is placed near other machinery making noise you can build noise as well. Isolating each piece of equipment can dramatically reduce the noise levels.
Why do I care about noise at my property line?
Noise Control is measured at your lot line. The neighbor’s home is maybe 400 feet from the lot line but codes state that you can not send noise off your property. Communities may or may not have a noise ordinance for you to follow. If they do not have an existing ordinance you can be assured they will write one directed at you.